The present invention relates generally to the investment casting art, and more specifically to a machinable pattern material composition adapted for use in the lost pattern process of precision investment casting.
In the lost pattern process for investment casting, bulk investment molds and ceramic shell molds are made using patterns which are replicas of the parts to be cast in metal. Bulk investment molds are prepared by placing an assembly of the patterns in a flask and filling the flask with a refractory investment slurry which is allowed to harden to form the mold. Ceramic shell molds are prepared by coating a pattern assembly with a refractory slurry, sanding or stuccoing the slurry coating while wet with coarser refractory material, and allowing the coating to harden, as by forced-air drying at room temperature, to form a thin layer of ceramic material having refractory particles embedded in the surface. After the first ceramic layer is sufficiently hard and dry, the steps of coating, draining, stuccoing and drying are repeated until a refractory shell having a desired thickness has been built up around the patterns.
The patterns used in both the bulk investment and the ceramic shell molding processes are made from a disposable material such that the patterns can be melted, as by heating in a furnace, oven, or autoclave, to remove them from the finished mold and provide the casting cavities. The usual pattern materials employed in production work are wax blends and mixtures of waxes and synthetic resins. Patterns made from such materials can be readily and economically molded using production tooling, e.g. wax or plastic injection equipment. The pattern waxes and resin formulations used for production work do not have good machining characteristics.
When only a small number of castings are required, as in prototype work which may involve the production of only one or a few castings for experimental purposes, it is usually faster and more economical to produce the patterns by machining operations instead of using production tooling. Polystyrene has been a conventionally employed material for prototype pattern purposes because it is readily machinable and can be handled without breaking. A significant disadvantage of making patterns from plastics such as polystyrene is that it is difficult to remove the patterns from the molds by heating without cracking the molds. When a mold containing a disposable pattern made of polystyrene is heated to melt or burn out the pattern material, severe internal pressures are created by the thermal expansion of the material. Ceramic shell molds normally do not have sufficient strength to resist these internal pressures caused by the heated pattern material. Even though bulk investment molds are considerably stronger than ceramic shell molds, some cracking of the molds during the pattern removal operation frequently occurs when pastic patterns are used.
An improved prototype pattern material comprising a fatty acid ketone and ethyl cellulose is disclosed in U.S. Pat. No. 3,296,006, issued Jan. 3, 1967. Extensive experience with compositions based on the teaching of this patent has confirmed that its machining properties are excellent and vastly superior to previous materials that have been used for machining prototypes, including polystyrene and other plastics. This experience has also confirmed that the occurrence of mold cracking during the pattern removal operation is markedly less when using pattern compositions based on ethyl cellulose and a fatty acid ketone than when using polystyrene patterns. On the other hand, it has been discovered that the ethyl cellulose-fatty acid ketone composition tends to produce more cracking problems than conventional pattern waxes. This fact has limited the use of the ethyl cellulose-fatty acid ketone material to the production of small patterns when a crack-free pattern removal operation is necessary or to require patching of the cracked molds when larger pattern sizes are attempted. In some instances involving very large patterns, mold cracking can be so severe as to make the use of ethyl cellulose-fatty acid ketone material impractical.